N4BP2L2 (NEDD4-binding protein 2-like 2), also known as PFAAP5 (phosphonoformate immuno-associated protein 5), is a 583 amino acid nuclear protein that potentially is involved in transcriptional regulation. N4BP2L2 is phosphorylated on Ser 199 in response to DNA damage, probably by ATM or ATR. Primarily expressed in bone marrow, N4BP2L2 is dramatically down-regulated after exposure to arsenic compounds, an event which precedes neutropenia. PFAAP5 interacts with both Gfi-1 and Neutrophil Elastase, two proteins that are implicated in neutropenia disorders. Defects in the gene encoding Neutrophil Elastase, ELA2, are the cause of cyclic haematopoiesis, which, with decreased numbers of circulating neutrophils, leads to an increased risk for opportunistic infection.